Planarization of non-planar surfaces in device fabrication

ABSTRACT

A method of planarizing a surface of a wafer includes providing a planarization material on the wafer surface and bringing a substantially flat surface into contact with the planarization material on the wafer. The planarization material is exposed to radiation at a first wavelength to cure the planarization material and is exposed to radiation at a second wavelength to cause changes to the planarization material that facilitate separation of the substantially flat surface from the planarization material.

BACKGROUND

The present invention relates generally to device processing and, in particular, to the planarization of non-planar surfaces in the fabrication of semiconductor or other devices.

The fabrication of electronic devices, such as semiconductor devices, typically produces surfaces that are non-planar. In some fabrication techniques, for example, multiple layers are formed sequentially on a silicon or other substrate. The layers can be processed into a desired pattern by the selective deposition of one or more materials or by removing selected regions of one or more layers. As more layers are formed, the topography or irregularity of the surface tends to increase.

An irregular surface can present various difficulties during device fabrication. For example, lithographic processes are often used to provide the patterning for the layers. During a typical lithographic process, a photosensitive material is deposited on a layer, exposed to radiation in a desired pattern, and developed to reveal the exposed pattern. To produce a photolithographic pattern with high resolution, the exposure light is focused at a specific depth, and the focus is maintained over a range of depths that is at least about two times the width of the pattern features. However, if the deposited resist material is not planar, the exposed image will not be in focus across the entire semiconductor wafer and throughout the film thickness. The likelihood of degraded lithographic results increases. Moreover, surface irregularities adversely affect metallization and other interconnections because the metal deposited over the surface has bends and turns which conform to the surface irregularities. Such bends and turns can cause undesirable current crowding.

To reduce the effects of surface irregularities, techniques are used to planarize the surface of the device on which the photosensitive material is deposited. Such techniques include, for example, etch-back techniques as well as chemical-mechanical polishing (CMP) techniques.

Recently, a technique has been proposed in which an object with a flat surface is pressed into contact with a planarization material, such as a photoresist or resin, deposited on the wafer. The planarization material is cured while in contact with the flat surface, and the flat surface is separated from the planarization material. The planar surface then can be transferred from the planarization material to the underlying layers.

One difficulty encountered with the foregoing technique is that, when the flat surface is separated from the planarization material, the planarization material can adhere to the flat surface, resulting in a surface that is less planar than desired. Accordingly, it would be advantageous to have a technique for planarizing a substrate in a manner that reduces or prevents adhesion of the planarization material to the flat surface.

SUMMARY

In general, according to one aspect, a method of planarizing a surface of a wafer includes providing a planarization material on the wafer surface and bringing a substantially flat surface into contact with the planarization material on the wafer. The planarization material is exposed to radiation at a first wavelength to cure the planarization material and is exposed to radiation at a second wavelength to cause changes to the planarization material that facilitate separation of the flat surface from the planarization material.

The flat surface can be provided, for example, by using an optical flat that is transparent to the radiation at the first and second wavelengths. The radiation at the first and second wavelengths can be transmitted through the optical flat and subsequently absorbed by the planarization material. The planarization material can include, for example, a resist or resin.

In various implementations, one or more of the following features are present. In one embodiment, curing of the planarization material is inhibited at or near the interface with the flat surface to facilitate separation of the planarization material and the flat surface. The planarization material can include an agent which is sensitive to the radiation at the second wavelength and which inhibits curing of the planarization material when exposed to the radiation at the second wavelength. Preferably, the radiation at the second wavelength is absorbed substantially by the planarization material at or near the interface between the planarization material and the flat surface. In one particular implementation, the planarization material includes an acid-generating agent which is sensitive to radiation at the first wavelength and a base-generating agent which is sensitive to radiation at the second wavelength. The base generated at or near the interface when the planarization material is exposed to radiation at the second wavelength can be used to neutralize acid produced when the planarization material is exposed to radiation at the first wavelength.

In a second embodiment, the planarization material can produce a gaseous by-product when exposed to radiation at the second wavelength. The gaseous by-product can produce localized pressure at the interface between the planarization material and the flat surface. The localized pressure can facilitate separation of the planarization material from the flat surface. In one particular implementation, the planarization material includes a carbonate which generates a gas such as carbon dioxide when exposed to deep ultraviolet radiation.

In a third embodiment, the planarization material can include an agent which is sensitive to the radiation at the second wavelength and which causes the planarization material to decompose when exposed to the radiation at the second wavelength. Preferably, the radiation at the second wavelength is absorbed substantially by the planarization material at or near the interface between the planarization material and the flat surface. For example, in one particular implementation, the planarization material can include a polymer with a doubly substituted carbon backbone and the second wavelength can be in the deep UV range.

The foregoing techniques can be combined to facilitate further separation of the planarization material from the optical flat.

In general, improved planarization is possible by using the techniques described herein. Higher degrees of planarization are made possible even for devices with severe topography.

Other features and advantages will be readily apparent from the following detailed description the, accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary assembly for use in planarizing a surface of a semiconductor device.

FIGS. 2-4 illustrate generally the planarization process according to the invention.

FIGS. 5A through 5C illustrate a technique according to a first embodiment of the invention.

FIGS. 6A and 6B illustrate a technique according to a second embodiment of the invention.

FIG. 7 illustrates a technique according to a third embodiment of the invention.

DETAILED DESCRIPTION

As described in greater detail below, an object with a substantially flat surface, such as an optical flat, is used to planarize a surface of a semiconductor or other device during the device fabrication process. The techniques described below relate to reducing or eliminating adhesion of a planarization material, such as a resin, to the optical flat.

In general, the planarization material includes a first optically sensitive agent which allows the planarization material to be cured or hardened by exposing it to a source of radiation at a first wavelength. The planarization material also includes a second agent which, when exposed to a source of radiation at a second wavelength, affects the properties of the planarization material to reduce or prevent adhesion to the flat surface of the object. The second wavelength can be chosen so that layers of the planarization material at or near the interface with the flat surface of the object are affected selectively, while the properties of the remaining planarization material remain substantially unaffected by the radiation at the second wavelength.

FIG. 1 illustrates an exemplary assembly 10 which includes an optical flat 12 having a lower surface 13 which can be brought into contact with a surface of a semiconductor or other wafer 14 to be planarized. The surface 13 should be sufficiently flat to planarize the wafer to at least sixty percent and, preferably, to at least eight-five percent. The optical flat 12 can be, for example, a fused silica disc that is transparent to the first and second wavelengths. Alternatively, the optical flat can be made of quartz or sapphire.

Initially, a planarization material 20 is applied in a flowable state to the top surface of the wafer 14 (FIG. 2). The wafer 14 can include one or more semiconductor layers or structures and can include active or operable portions of semiconductor devices. Additional layers of other materials also may be present. The wafer 14 is placed on a flexible mat 16 above a pressurizing chamber 18 with the surface of the wafer that is to be planarized facing upward. The chamber 18 is pressurized to press the planarization material 20 on the wafer 14 into contact with the optical flat 12 so as to transfer the flatness of the optical flat to the planarization material (FIG. 3). In some implementations, prior to moving the optical flat 12 into contact with the planarization material 20, the planarization material is soft cured at an elevated temperature to remove solvent. Once the optical flat 12 is in contact with the planarization material, the planarization material is exposed to radiation of at least two different wavelengths or wavelength ranges. One wavelength λ₁ is selected to cure or harden the planarization material, and the second wavelength λ₂ is chosen to affect properties of the planarization material so as to facilitate separation of the lower surface 13 from the planarization material. The optical flat 12 then is separated from the planarization material, leaving a planarized surface on the wafer (FIG. 4). The planarity of the planarization material can be transferred to the underlying layer(s) on the wafer 14 using known techniques, for example, by etching the planarization material 20 and a portion of the underlying layer(s).

Various planarization materials that are solidified by curing include an acid-generating additive that generates acid when heated or irradiated. The acid catalyzes polymerization. Such acid-generating additives are known to those of ordinary skill in the art, and the specific acid-generating additive used depends on the planarization material and the desired curing conditions. In general, the acid-generating additive is selected to be sensitive to radiation at the first wavelength λ₁, which, in some implementations, is in the visible or near ultraviolet (near UV) range. For example, in some implementations, the first wavelength λ₁ is selected to be approximately 400 nanometers (nm) or longer. A phenolic polymer or epoxy resin, such as epoxy novolac 431 (DEN-431) which is commercially available from the Dow Chemical Co., can be used as the planarization material 20. Other phenolic resins, such as hydroxystryrene polymers, also can be used. A suitable acid catalyst is the photoacid generating additive triphenyl-sulphonium hexafluoroantimonate.

According to a first embodiment, a base generating additive also is added to the planarization material 20. As explained below, one purpose of adding the base-generating additive is to inhibit curing of the planarization material 20 near the interface with the optical flat 12. The base generating additive should be sensitive to radiation at a second wavelength λ₂, yet inert or substantially inert to radiation at the first wavelength λ₁. Moreover, the second wavelength λ₂ should be selected so that radiation at the second wavelength is primarily absorbed near the surface of the planarization material 20 at the interface with the optical flat 12 and does not penetrate very far into the planarization material. For example, in some implementations, a base generating additive that is sensitive to radiation having a wavelength λ₂ in the deep UV range, in other words, radiation having a wavelength in the range of about 190-280 nm, can be used. Examples of such base-generating additives include bis{[(2,6-dinitro-benzyl)oxy]carbonyl}hexane-1,6-diamine, which are sensitive to radiation at wavelengths of about 254 nm and lower. The penetration depth of such radiation in phenolic polymers such as epoxy novolac431 is only about 30-50 nm.

According to the first embodiment, a planarization material 20 including an acid-generating photo-agent and a base-generating photo-agent is deposited onto the surface of the wafer 14 which then is placed onto the flexible mat 16. The optical flat 12 is brought into contact with the planarization material 20. The planarization material 20 is then exposed to the radiation at the first wavelength λ₁ and the second wavelength λ₂ at substantially the same time. Alternatively, the planarization material 20 can be exposed to the radiation at the second wavelength λ₂ and subsequently to the radiation at the first wavelength λ₁.

Exposure of the planarization material 20 to radiation at the second wavelength λ₂ produces an excess of base near the interface with the optical flat 12. The excess base serves to neutralize the acid that is produced by exposure of the planarization material to radiation at the first wavelength λ₁, thereby inhibiting the acid from curing the planarization material. However, since the radiation at the second wavelength λ₂ has a shallow penetration depth into the planarization material 20, the base produced by that radiation only inhibits curing of the planarization material at or near the interface with the optical flat 12. The remainder of the planarization material 20 is cured by exposure to the longer wavelength radiation (λ₁) which penetrates throughout the planarization material. The technique is shown schematically in FIGS. 5A through 5C. Inhibiting curing of the planarization material 20 at or near the interface with the optical flat 12 reduces adhesion between the two surfaces and facilitates separation of the optical flat from the planarization material.

In some implementations, a single source of radiation can be used to provide radiation at the first wavelength λ₁ and the second wavelength λ₂. For example, in one implementation, a mercury arc lamp with appropriate filters is used. Alternatively, separate radiation sources can be used. Thus, for example, a deuterium lamp or an excimer laser can be used to provide radiation at the second, shorter wavelength λ₂.

According to a second embodiment, the planarization material 20 can include a photosensitive agent which, when exposed, for example, to deep UV radiation, produces one or more gases such as hydrogen (H₂), nitrogen (N₂) , nitrous oxide (N₂O) , sulfur tri-oxide (SO₃), acetylene (C₂H₂), carbon dioxide (CO₂), ammonia (NH₃) or methane (CH₄). Radiation at a first wavelength λ₁, such as visible or near UV, can be used to cure the planarization material, and the deep UV radiation (λ₂) can be used to produce one or more of the foregoing gases (FIGS. 6A and 6B). The generation of the gas(es) produces localized pressure near the interface between the planarization material 20 and the optical flat 12 to facilitate separation of the optical flat from the planarization material.

In one implementation, for example, a carbonate such as (HOC₂H₄NH₃)₂CO₃ is added to a phenolic polymer such as epoxy novolac 431. By tuning the deep UV radiation to break the bonds of the carbonate, the carbonate decomposes and produces CO₂ as a by-product:

(HOC₂H₄NH₃)₂CO₃→CO₂+H₂O+(HOC₂H₄NH₂).

The radiation which causes the carbonate to decompose and release the CO₂ gas can be provided either at the same time as the curing radiation or subsequent to the curing radiation.

The technique of the first embodiment, in which curing of the planarization material is inhibited near the interface with the optical flat, and the technique of the second embodiment, in which gaseous by-products are produced to provide localized pressure, can be used together to facilitate separation of the optical flat 12 from the planarization material 20.

According to a third embodiment, a polymer, such as poly α methyl, 3, hydroxy-6-methyl, styrene, is provided with doubly substituted carbon backbone atoms and is used as the planarization material 20. As explained below, one purpose of providing the doubly substituted carbon backbone is to allow the planarization material 20 to decompose near the interface with the optical flat 12 when exposed to radiation at a second wavelength during or following curing. In particular, polymers with a doubly substituted carbon backbone are sensitive to deep UV radiation and react by scission at the substituted carbon atom, rather than crosslinking. Moreover, the deep UV radiation is absorbed near the upper surface of the polymer and does not penetrate deeply into the material. Other functional groups which facilitate the photo-decomposition of the polymer also can be used.

In the third embodiment, a polymer with a doubly substituted carbon backbone is deposited on the wafer 14 and serves as the planarization material 20. After the optical flat 12 is brought into contact with the planarization material, the planarization material is exposed to radiation at a first wavelength λ₁ (e.g., greater than 400 nm) and radiation at the second wavelength λ₂ in the deep UV range. Radiation at the first wavelength serves to cure the planarization material 20. When the planarization material 20 is exposed to the deep UV radiation, scission occurs at the substituted carbon atoms. Since the deep UV radiation does not penetrate deeply into the planarization material 20, the polymer decomposes only near the interface with the optical flat 12 (FIG. 7). The decomposed surface of the planarization material facilitates separation from the optical flat 12.

The techniques of the second and third embodiments can be used together to facilitate separation of the optical flat 12 from the planarization material 20.

Although the invention has been described with respect to transferring a substantially flat or planar surface to surfaces underlying the resist, the techniques described above also can be used to transfer a curved or other non-planar surface to the underlying layers.

Other implementations are within the scope of the following claims. 

What is claimed is:
 1. A method of planarizing a surface of a wafer, the method comprising: providing a planarization material on the wafer surface; bringing a substantially flat surface into contact with the planarization material on the wafer surface; exposing the planarization material to radiation at a first wavelength to cure the planarization material; and exposing the planarization material to radiation at a second wavelength to cause changes to the planarization material that facilitate separation of the substantially flat surface from the planarization material.
 2. The method of claim 1 wherein the planarization material includes an agent which is sensitive to the radiation at the second wavelength and which inhibits curing of the planarization material when exposed to the radiation at the second wavelength.
 3. The method of claim 2 wherein the planarization material includes a phenolic polymer and wherein the second wavelength is in a deep UV range.
 4. The method of claim 2 wherein the radiation at the second wavelength is absorbed substantially by the planarization material within approximately 50 nanometers of an interface between the planarization material and the substantially flat surface.
 5. The method of claim 4 wherein bringing the substantially flat surface into contact with the planarization material includes moving an optical flat into contact with the planarization material and wherein the radiation at the first and second wavelengths passes through the optical flat.
 6. The method of claim 2 wherein the steps of exposing the planarization material to radiation at the first and second wavelengths take place substantially at the same time.
 7. The method of claim 2 wherein exposing the planarization material to radiation at the second wavelength takes place prior to exposing the planarization material to radiation at the first wavelength.
 8. The method of claim 1 wherein the planarization material includes a first agent which is sensitive to radiation at the first wavelength greater than about 400 nanometers and a second agent which is sensitive to radiation at the second wavelength in the range of about 190-280 nanometers and which inhibits curing of the planarization material when exposed to the radiation at the second wavelength.
 9. The method of claim 1 wherein the planarization material includes an acid-generating agent which is sensitive to radiation at the first wavelength and a base-generating agent which is sensitive to radiation at the second wavelength.
 10. The method of claim 9 wherein the first wavelength is longer than the second wavelength.
 11. The method of claim 1 including bringing an optical flat into contact with the planarization material, wherein the optical flat is transparent to the radiation at the first and second wavelengths, and wherein exposing the planarization material to radiation at the first and second wavelengths includes passing the radiation at the first and second wavelengths through the optical flat.
 12. The method of claim 1 wherein the planarization material includes an agent which is sensitive to the radiation at the second wavelength and which causes the planarization material to decompose when exposed to the radiation at the second wavelength.
 13. The method of claim 12 wherein the planarization material includes a polymer with a doubly substituted carbon backbone.
 14. The method of claim 12 wherein the steps of exposing the planarization material to radiation at the first and second wavelengths take place substantially at the same time.
 15. The method of claim 12 wherein exposing the planarization material to radiation at the first wavelength takes place prior to exposing the planarization material to radiation at the second wavelength.
 16. The method of claim 15 wherein the second wavelength is in a deep UV range.
 17. The method of claim 1 wherein the planarization material produces a gaseous by-product when exposed to radiation at the second wavelength.
 18. The method of claim 17 wherein the planarization material includes a carbonate which produces the gaseous by-product when exposed to radiation at the second wavelength.
 19. The method of claim 17 wherein the gaseous by-product is selected from a group consisting of hydrogen (H₂), nitrogen (N₂), nitrous oxide (N₂O), sulfur tri-oxide (SO₃), acetylene (C₂H₂), carbon dioxide (CO₂), ammonia (NH₃) and methane (CH₄).
 20. The method of claim 17 wherein the gaseous by-product produces localized pressure at an interface between the planarization material and the substantially flat surface.
 21. The method of claim 1 further including: separating the substantially flat surface from the planarization material.
 22. A method of planarizing a surface of a wafer, the method comprising: providing a planarization material on the wafer surface, wherein the planarization material includes a first acid-generating agent sensitive to radiation at a first wavelength and a second base-generating agent sensitive to radiation at a second wavelength, wherein the planarization material is cured in the presence of acid; bringing a substantially flat surface into contact with the planarization material on the wafer; exposing the planarization material to radiation at the second wavelength to produce base near an interface between the planarization material and the substantially flat surface; and exposing the planarization material to radiation at the first wavelength to produce acid in the planarization material.
 23. The method of claim 22 wherein the planarization material further includes a phenolic polymer, and wherein the first wavelength is greater than the second wavelength.
 24. The method of claim 22 wherein the radiation at the second wavelength is substantially entirely absorbed by the planarization material within approximately 50 nanometers of the interface between the planarization material and the substantially flat surface.
 25. The method of claim 24 wherein the base neutralizes the acid to inhibit curing of the planarization material near the interface between the planarization material and the substantially flat surface.
 26. The method of claim 22 further including: separating the substantially flat surface from the planarization material. 